Vacuum IG units are known in the art. For example, see U.S. Pat. Nos. 5,664,395, 5,657,607, and 5,902,652, the disclosures of which are all hereby incorporated herein by reference.
FIGS. 1-2 illustrate a conventional vacuum IG unit (vacuum IG unit or VIG unit). Vacuum IG unit 1 includes two spaced apart glass substrates 2 and 3, which enclose an evacuated or low pressure space 6 therebetween. Glass sheets/substrates 2 and 3 are interconnected by peripheral or edge seal of fused solder glass 4 and an array of support pillars or spacers 5.
Pump out tube 8 is hermetically sealed by solder glass 9 to an aperture or hole 10 which passes from an, interior surface of glass sheet 2 to the bottom of recess 11 in the exterior face of sheet 2. A vacuum is attached to pump out tube 8 so that the interior cavity between substrates 2 and 3 can be evacuated to create a low pressure area or space 6. After evacuation, tube 8 is melted to seal the vacuum. Recess 11 retains sealed tube 8. Optionally, a chemical getter 12 may be included within recess 13.
Conventional vacuum IG units, with their fused solder glass peripheral seals 4, have been manufactured as follows. Glass frit in a solution (ultimately to form solder glass edge seal 4) is initially deposited around the periphery of substrate 2. The other substrate 3 is brought down over top of substrate 2 so as to sandwich spacers 5 and the glass frit/solution therebetween. The entire assembly including sheets 2, 3, the spacers, and the seal material is then heated to a temperature of approximately 500° C., at which point the glass frit melts, wets the surfaces of the glass sheets 2, 3, and ultimately forms hermetic peripheral or edge seal 4. This approximately 500° C. temperature is maintained for from about one to eight hours. After formation of the peripheral/edge seal 4 and the seal around tube 8, the assembly is cooled to room temperature. It is noted that column 2 of U.S. Pat. No. 5,664,395 states that a conventional vacuum IG processing temperature is approximately 500° C. for one hour. Inventors Lenzen, Turner and Collins of the '395 patent have stated that “the edge seal process is currently quite slow; typically the temperature of the sample is increased at 200° C. per hour, and held for one hour at a constant value ranging from 430° C. and 530° C. depending on the solder glass composition.” After formation of edge seal 4, a vacuum is drawn via the tube to form low pressure space 6.
Unfortunately, the aforesaid high temperatures and long heating times of the entire assembly utilized in the formulation of edge seal 4 are undesirable, especially when it is desired to use a heat strengthened or tempered glass substrate(s) 2, 3 in the vacuum IG unit. As shown in FIGS. 3-4, tempered glass loses temper strength upon exposure to high temperatures as a function of heating time. Moreover, such high processing temperatures may adversely affect certain low-E coating(s) that may be applied to one or both of the glass substrates in certain instances.
FIG. 3 is a graph illustrating how fully thermally tempered plate glass loses original temper upon exposure to different temperatures for different periods of time, where the original center tension stress is 3,200 MU per inch. The x-axis in FIG. 3 is exponentially representative of time in hours (from 1 to 1,000 hours), while the y-axis is indicative of the percentage of original temper strength remaining after heat exposure. FIG. 4 is a graph similar to FIG. 3, except that the x-axis in FIG. 4 extends from zero to one hour exponentially.
Seven different curves are illustrated in FIG. 3, each indicative of a different temperature exposure in degrees Fahrenheit (° F.). The different curves/lines are 400° F. (across the top of the FIG. 3 graph), 500° F., 600° F., 700° F., 800° F., 900° F., and 950° F. (the bottom curve of the FIG. 3 graph). A temperature of 900° F. is equivalent to approximately 482° C., which is within the range utilized for forming the aforesaid conventional solder glass peripheral seal 4 in FIGS. 1-2. Thus, attention is drawn to the 900° F. curve in FIG. 3, labeled by reference number 18. As shown, only 20% of the original temper strength remains after one hour at this temperature (900° F. or 482° C.). Such a significant loss (i.e., 80% loss) of temper strength is of course undesirable.
In FIGS. 3-4, it is noted that much better temper strength remains in a thermally tempered sheet when it is heated to a temperature of 800° F. (about 428° C.) for one hour as opposed to 900° F. for one hour. Such a glass sheet retains about 70% of its original temper strength after one hour at 800° F., which is significantly better than the less than 20% when at 900° F. for the same period of time.
Another advantage associated with not heating up the entire unit for too long is that lower temperature pillar materials may then be used. This may or may not be desirable in some instances.
Even when non-tempered glass substrates are used, the high temperatures applied to the entire VIG assembly may soften the glass or introduce stresses, and partial heating may introduce more stress. These stresses may increase the likelihood of deformation of the glass and/or breakage.
Moreover, VIG units are subject to extremely large static and dynamic loading as well as thermally induced stresses both during its manufacturing (e.g., during pump down and thermal seal processing) and throughout its service life (e.g., during wind-loads or mechanical and thermal shocks). The pillar spacers used to mechanically support the gap between the two substrates tend to indent the glass surfaces with which they in contact, thereby creating indented areas from which cracks may propagate and hence weakening the glass structure. The glass region just above the pillar is under compressive stress, whereas the peripheral region of the pillar is under tensile stress. It has been found that it is in the tensile regime that annealed glass is at its weakest state, and it has been found that any surface and bulk flaws in the tensile stress field may develop into cracks that may propagate. The magnitude of the tensile stress component increases with the inter-pillar spacing, and the likelihood of the cracks forming and ensuing catastrophic breakage increases once the stress field is above the strength of the glass. The surface profile or contour of the pillar is related to the likelihood of any kind of Hertzian or coin shaped cracks.
One way to mitigate the indentation crack issue (e.g., while still being aggressive on pillar spacing) is to use glass that has been tempered such that the surface skin of the glass is in a highly compressive stress that tends to “wash out” the tensile stress components induced by supporting pillars. Unfortunately, however the VIG process takes place at high temperatures and involves a thermal cycle duration that potentially can de-temper the glass.
Thus, it will be appreciated that there is a need in art to find a solution to the problems of and associated with indentation cracking. It also will be appreciated that there is a need in the art for improved VIG units, and/or methods of making the same.
In certain example embodiments of this invention, a vacuum insulated glass (VIG) unit is provided. First and second substantially parallel, spaced apart glass substrates define a gap therebetween. An edge seal is provided around a periphery of the first and second substrates to form an hermetic seal. A plurality of pillars is provided between the first and second substrates. A lamellar coating is provided around at least a portion of the pillars so as to impart closure stresses on the first and/or second substrates proximate to the pillars to at least partially offset tensile stresses also applied to the first and/or second glass substrates. The gap is provided at a pressure less than atmospheric.
In certain example embodiments of this invention, a vacuum insulated glass (VIG) unit is provided. First and second substantially parallel, spaced apart glass substrates define a gap therebetween. An edge seal is provided around a periphery of the first and second substrates to form an hermetic seal. A plurality of pillars is provided between the first and second substrates. A high-aspect ratio lamellar coating is provided between said pillars and the first and/or second substrates so as to at least partially offset tensile stresses applied to the first and/or second glass substrates. The gap is provided at a pressure less than atmospheric.
In certain example embodiments of this invention, a method of making a vacuum insulated glass (VIG) unit is provided. First and second glass substrates are provided. A plurality of pillars are provided on the first glass substrate. The first and second substrates are sealed together (e.g., using at least one edge seal) such that the first and second substrates are in substantially parallel, spaced apart orientation to one another and to define a gap therebetween. The gap is evacuated to a pressure less than atmospheric. A lamellar coating is provided between the pillars and the first and/or second substrates so as to at least partially offset tensile stresses also applied to the first and/or second glass substrates.
In certain example embodiments of this invention, a VIG unit is provided. First and second substantially parallel, spaced apart glass substrates define a gap therebetween. An edge seal is provided around a periphery of the first and second substrates to form a seal. A plurality of pillars is provided between the first and second substrates, with each said pillar being at least partially laminated with a material selected to impart closure stresses on the first and/or second substrates proximate to the pillar to at least partially offset tensile stresses also applied to the first and/or second glass substrates. The gap is provided at a pressure less than atmospheric.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.